1. Field of Use
These teachings relate generally to a fluorescent color calibration apparatus and more particularly to providing a fluorescent color standard or reference within photographic or imaged areas for calibrating RGB values.
2. Description of Prior Art (Background)
The photographic documentation of fluorescence in various fields, including art conservation, forensics, gemology and medicine, lacks a standard for calibrating fluorescent color balance. As a result, the interpretation of fluorescent colors is highly subjective and, therefore, the potential evidentiary value of photographic documentation of fluorescence is limited. Numerous variables such as the intensity of the UV radiation, the distance between the UV radiation source and the fluorescing materials, wavelength variations in UV radiation sources and camera specific variables (such as white balance), make the meaningful comparison between one set of UV photographic documentation and another set of UV photographic documentation nearly impossible. In addition, all light sources currently used in fluorescent photography lack stability over long periods. This lack of stability causes the output of a xenon lamp, for example, to fluctuate as a function of time, which affects fluorescence intensity of a given photograph, all other conditions being equal. To perform accurate and comparable photographic fluorescent color analyses, these light source fluctuations must be taken into account. In general, the intensity from a UV lamp is quite consistent. Unlike a tungsten bulb which can be dimmed to lower the intensity, a UV lamp has the same UV output. However, intensity can be adjusted intentionally, or unintentionally, in two ways: one is to move the lamp away or closer to the object; the other is use a higher or lower wattage UV bulb.
There is no prior product available on the market though there is some haphazard use of various florescent materials the might be familiar to the end user. A ruler is available through the Lighting Powder Company LLC which is advertised for forensic documentation. The ruler's orange ink exhibits sections of different fluorescent intensity when exposed to UV sources. While somewhat useful for recording the intensity of florescence, use of the ruler for color corrections is not possible. In addition, the high fluorescence end of the ruler actually produces an orange light that can skew the color balance; especially of images made to record low/moderate fluorescence. Likewise, there are fluorescent rulers primarily used for forensic investigation. These rulers will fluoresce (typically an orange color) under long wave ultraviolet radiation. The ruler also contains different sections to indicate the intensity of the fluorescence and can be used to indicate whether short wave ultraviolet was used as the excitation source. Such devices are limited in that the fluorescence is not consistent and can not be correlated to a range of measured or standard values. Therefore these devices are ineffective for color correcting recorded fluorescence.
Past practice also relied on a few known fluorescent colors, such as shellac, rose madder, or zinc white which are used as reference colors. These fluorescent colors are not consistent due to natural variability and manufacturing methods. Also the range of color offered by these materials covers only off-white, pink, and orange.
Other approaches rely on including various fluorescence materials into a photographic image documenting fluorescence. Materials like zinc white, rose madder, or shellac have characteristic fluorescence behaviors that those skilled in the field will visually recognize. However, the fluorescent properties of these materials vary greatly from one source to another and thus cannot be used for accurate calibration. Fluorescent “rulers” can provide a qualitative sense of the intensity of the fluorescence but not color information. Typically, the color and intensity of UV fluorescence is corrected, if at all, “by eye”, thus undermining the evidentiary value of such documents.
Also, the intensity of fluorescence also varies greatly from one material to another. Most naturally occurring organic materials have some fluorescence, though this fluorescence can be very low. On the other hand, many synthetic materials have extremely high levels of fluorescence. Use of a single standard for both very bright fluorescence and dim fluorescence is unacceptable due to the sensitivity range of both the traditional silver halide emulsions (analog photographic) and/or digital cameras. If one uses only one reference card (with a fixed brightness) then it might not be possible to record fluorescent colors (both from the object and reference card) with correct color saturation if the object and the reference card have significantly different brightness. If the object has strong fluorescence, like a painting painted with day-glow pigments, the correct exposure time for the camera needs to be short. In this case a reference card might appear too dark due to underexposure. If the reference card is too dark (or too light) then the measured fluorescent values are not applicable and the image cannot be color corrected. Therefore, to compensate for fluorescent intensity differences, two or more fluorescent color standards are needed to more precisely capture the fluorescence behavior of materials with high and low fluorescent intensity.
It will be appreciated that there is no known existing standard for calibrating or adjusting a photographic record of visible fluorescence colors. In addition, single standards, such as white standards (made of barium sulfate) used to calibrate UV spectrophotometers are expensive as well as hazardous.